Late 20th century hottest in over 1000 years

Monday, 22 April 2013 Bianca NogradyABC

Heat wave: Previous understanding of climate change has been based largely on the northern hemisphere, but this study covers change in the southern hemisphere as well(Source: anniegreenwood/iStockphoto)

Record heat Average temperatures around the world in the last thirty years of the 20th century were higher than any other time in nearly 1400 years.

That's the conclusion of the first climate reconstruction to examine global climate change from a regional perspective by an international network of climatologists known as the PAGES 2k network.

Their findings, based on climate data from eight continental scale regions, including Australasia, Europe, North and South America, are published today in Nature Geoscience .

The data, which was derived from sources such as tree rings, glacier ice, pollen and corals, showed all regions except Antarctica experienced a long-term cooling trend that reversed abruptly in the 20th century.

"Excluding Antarctica, the 20th century average temperature among the six regions was about 0.4°C higher than the averaged temperatures of the preceding five centuries," write the authors.

"Compared to the preceding five centuries, 20th-century warming in the four northern hemisphere regions was, on average, about twice that of the more strongly ocean-dominated regions of Australasia and South America (about 0.5°C compared with 0.2°C), with the greatest differences at northern high latitudes."

This is the first major effort to examine regional climate variability over the past two thousand years, says co-author and paleoclimatologist Dr Steven Phipps.

"Previous understanding of changes over this period have been based largely on the northern hemisphere because that's where most of the records are and that's where most of the scientists are, but what we're doing here is extending it to the southern hemisphere as well," says Phipps, research fellow at the University of New South Wales.

Past regional differences

One surprising discovery was that past changes in global climate didn't necessarily happen at the same time around the world.

There were no globally synchronous multi-decadal warm or cold intervals that define a Medieval Warm Period or Little Ice Age, although conditions were generally cold between AD 1580 and 1880, punctuated in some regions by warm periods in the 18th century, the researchers report.

"What we thought of in the past as being globally uniform phenomena, such as the Medieval Warm Period and the Little Ice Age, didn't happen at the same time -- for example, the medieval warming happened earlier in the northern hemisphere than the southern hemisphere," Phipps says.

This discovery addresses a long-running issue within climatology, according to paleoclimatologist Associate Professor Ian Goodwin, who was not involved in the study.

"When I started working in Antarctica in the 1980s, that view still prevailed and we were expecting that there were relatively synchronous changes in climate between all regions around the globe," says Goodwin, from Macquarie University.

However since that time it has become apparent that regional temperature records disagree, prompting concerns that methodology or the data were somehow flawed.

"Really in the last ten years it has become more and more evident that we were looking for something that didn't exist" Goodwin says.

20th century warming unusual

But while the study revealed that past global climate phenomena have occurred at different times in different regions, the data also shows that 20th century warming is unusual in being globally synchronous.

"Prior to the 20th century natural drivers were dominant, such as change in solar output and volcanic eruption, however what happened during the 20th century is that human influences and predominantly greenhouse gases become dominant," Phipps says.

However there is still a slight difference between northern and southern hemispheres due to the greater land mass of the northern hemisphere, which means the southern hemisphere is warming more slowly.